by Kelsey Mittauer | CBS 11 Investigative ProducerBy Ginger Allen

FORT WORTH (CBSDFW.COM) – It’s the kind of video we see time and time again: suspects committing crimes in masks or disguises. But one thing they can’t hide: the way they walk.

That’s where forensic podiatry comes into play.

The practice involves analyzing and comparing things like footwear and gait, which is how a person walks.

Dr. Michael Nirenberg runs a foot clinic in Indiana.

When he’s not with patients, he’s an expert witness in criminal cases across the country.

“When I look at a piece of footwear I’ll do what I call a ‘shoe autopsy,’ looking at the bottom of it, the wear pattern, the insole impression,” he said. “We can show that you wore the shoe and you were the only one who wore the shoe.”

Dr. Michael Nirenberg (CBS 11)

He’s helped convict and exonerate people using gait analysis, but he’s careful to point out that such findings should not be the only evidence used against someone.

He says more than one person can walk the same way, so it is just one piece of the puzzle. Nirenberg says he would not participate in a prosecution if the only evidence was gait analysis.

A few years ago, the FBI asked him to study the video of the suspect in the Missy Bevers murder investigation.

Bevers was killed in the early morning hours of April 18, 2016 as she was preparing to lead fitness class at Creekside Church in Midlothian.

“When I took this case on, I had a police officer put on similar armor and walk with – and without – the armor,” he said. “I put the armor on as well, just to see how I felt it would change how I walked.”

Dr. Nirenberg says wearing that gear and carrying a weapon would affect gait, so it’s difficult to know if that’s how the killer would move in regular clothing.

And while much has been made of the way the suspect walks, he says, it’s not uncommon.

“You can see how the left toe is slightly out-toed and the right toe is significantly out-toed in this person,” he said. “A large number of people in the population are going to have that.”

Another big question in this case is gender: police aren’t sure if the killer is a man or a woman.

Dr. Nirenberg says, gait can’t answer that question.

“You can’t make a determination if it’s a male or female because of variation in gait,” he told the CBS 11 I-Team’s Ginger Allen. “That is a dangerous thing to do.”

The church surveillance video is not the only video Dr. Nirenberg studied.

He says investigators on the case sent him recordings of people walking, to see if he thought they moved like the suspect.

“The FBI sent me persons who were of both genders,” he said. “I don’t know the people they sent me – who they were. I don’t believe they were being recorded.”

Dr. Nirenberg said just one of them had a gait similar to that of the suspect.

He says the FBI thanked him for his time and said they’d be in touch if they developed new evidence.

That hasn’t happened, but he says over the years amateur detectives have contacted him online with their own leads.

“Now and then people will send me video of their neighbors or people in the area and they’ll say ‘could this be the killer, the perpetrator?'” He says he always refers them to the police and advises them against surveilling people without their knowledge.

Other than those occasional reminders, Dr. Nirenberg says the one thing about this case that he can’t forget is the video never seen by the public.

He saw Missy Bevers inside the church before her murder.

He says she turned her head, appearing to hear something off-camera. It was very likely the moment she realized she was not alone.

“It sticks with you because you know what’s coming,” he said. “We’ve all watched those horror movies where you say ‘you gotta get out of the house’ and this was the real thing.”